1. Field of the Invention
This invention relates to a process for manufacturing lower cost molten carbonate fuel cell matrices which combines chemical synthesis or sol-gel powder synthesis techniques with tape casting or doctor blading. The chemical synthesis techniques enable the use of low cost precursor materials for the production of the high-purity, high surface area lithium aluminate matrices required for high performance molten carbonate fuel cells at a fraction of the cost of a vendor-provided lithium aluminate powder. Unlike traditional powder synthesis techniques which require complete solvent removal and a heat treatment and/or grinding procedure prior to use, in the process of this invention, the solvent utilized during the chemical or sol synthesis is also utilized as the tape casting solvent.
2. Description of Prior Art
Fuel cell testing in the United States and abroad has proven that high surface area, greater than about 10-15 m.sup.2 /g, low density, less than about 1.0 g/cm.sup.3, lithium aluminate (LiAlO.sub.2) is required to ensure submicron porosity and phase stability, characteristics required to ensure sufficient carbonate retention for maximum fuel cell performance and endurance. Due to the requirements for material purity and surface area, until now, a high cost alpha- or gamma-phase powder has been the only alternative for ensuring cell performance and endurance.
State-of-the-art molten carbonate fuel cell matrices are typically formed from expensive vendor LiAlO.sub.2 powder by tape casting. Tape casting involves suspending composite materials and a binder in aqueous or organic solvents and pouring the suspension into a doctor blade reservoir system. A blade opening is provided at the bottom of the reservoir and the slip is cast to a uniform height onto a moving substrate. A second blade provides improved dimensional control of the cast tape. The cast suspension passes through a drying section where the solvents evaporate, leaving behind a porous composite. Tape casting is taught, for example, by U.S. Pat. No. 5,473,008 which also teaches a casting composition comprising a ceramic powder, an organic solvent, a binder, a plasticizer and dispersant, the dispersant being a polyvinyl alcohol/fatty acid ester, and U.S. Pat. No. 5,453,101.
Gel casting refers to a ceramic forming process in which a slurry of ceramic powders in a solution of organic monomers is cast in a mold. The monomer mixture is polymerized in situ to form gelled parts. See Omatete et al., "Gel Casting--A New Ceramic Forming Process," Ceramic Bulletin, Vol. 70, No. 10, pp. 1641-1649. In the sol-gel processing of ceramics and glass, ceramic materials are formed through a low-temperature chemical synthesis during which a sol is formed from precursor materials. The solution is polymerized into a gel structure and heat treated to form the ceramic or glass powder. See Johnson, Jr., "Sol-Gel Processing of Ceramics and Glass," Ceramic Bulletin, Vol. 64, No. 12 (1985); Pierre, "Sol-Gel Processing of Ceramic Powders," Ceramic Bulletin, Vol. 70, No. 8, (1991), pp. 1281-1288; and Yoldas, "Alumina Sol Preparation From Alkoxides," Ceramic Bulletin, Vol. 54, No. 3 (1975), pp. 289-290. See also, U.S. Pat. No. 5,316,695 which teaches a polymeric catalyst, such as poly(styrene)sulfonic acid, which can be used in the synthesis of sol-gel derived ceramic materials from metal alkoxides by reaction of the metal alkoxides and a reactive endcapped polymeric modifier.
A variety of researchers have utilized various chemical precursors to manufacture lithium aluminate powders and fibers of various lithium aluminate phases. In all cases, however, a powder is the end product which is formed in a separate manufacturing procedure into the molten carbonate fuel cell matrix. See for example, Watanabe et al., "Crystal Growth of Rod-Shaped Beta-LiAlO.sub.2," Journal of American Ceramic Society, 70 (10), C268-269 (1987); Kinoshita et al., "Preparation and Characterization of Lithium Aluminate," Material Research Bulletin, 13, 445-455 (1978); and Kinoshita et al., "Synthesis of Fine Particle Size Lithium Aluminate For Application in Molten Carbonate Fuel Cells," Material Research Bulletin, Vol. 14, (1979), pp. 1357-1368. U.S. Pat. No. 5,252,315 teaches a process for producing lithium aluminate powder by contacting a raw material powder of a lithium aluminate with water to form a hydrate, followed by dehydration by heating. U.S. Pat. No. 5,545,427 teaches a process for preparing lithium aluminosilicate or gamma lithium aluminate ceramics in which a short chain anhydrous alcohol is mixed with a liquid unpolarized aluminum alkoxide to which water is added for hydrolyzing. The resulting mixture is dried at a temperature below 300.degree. C. to evaporate the alcohols and water and obtain a crystalline powder having a structure identical to that of beta lithium aluminate. The powder is then shaped by isostatic or non-isostatic cold pressing, by pouring a slip, by spinning, or by extruding, and subsequently sintered at a temperature of about 800.degree. to 1200.degree. C. Finally, U.S. Pat. No. 5,432,138 teaches a process for producing gamma lithium aluminate matrix layers for molten carbonate fuel cells using a slurry, the liquid phase of which is formed by an aqueous polyvinyl alcohol solution.